Squid-Imposter makes it easy to create Squid based proxy injecting your own content to chosen website URLs. Modified content is then persisted in client’s browser even when the client no longer connects through your proxy thanks to HTML5 Offline cache features. It’s a MITM/sidejacking attack technique by pretending to be that website. Injected content may for example be used to form a phishing attack during, it is a port of HTML5 offline cache poisoning features introduced in Imposter.
Source: https://thehackernews.com/2010/12/squid-imposter-squid-based-sidejacking.html